1. Field of the Invention
The present invention relates generally to an actively controlled suspension system which has adjustable suspension characteristics according to the vehicle driving condition. More specifically, the invention relates to an actively controlled suspension system which has adjustable rolling stability and/or pitching stability.
2. Description of the Background Art
Generally, a typical construction of a suspension assembly comprises a suspension coil and shock absorber and is interposed between a vehicle body and a suspension member supporting a vehicular wheel, in order to constitute an automotive suspension system with a suspension member. The suspension coil spring generally resists load applied to maintain the vehicle body and the road wheel in a predetermined positional relationship to each other. On the other hand, the shock absorber is intended to damp or absorb vibrations transmitted between the vehicle body and the road wheel. The automotive suspension system may be further provided with a roll-stabilizer for suppressing vehicular rolling motion.
Some of the many kinds of automotive suspension systems include hydraulic circuits associated with fluid chambers in hydraulic shock absorbers for the controlling balance between the fluid pressures in the fluid chambers according to the relative displacement between the vehicle body and road wheel. Such hydraulic circuits include a fluid pressure source supplying working fluid at a given pressure to the circuits, and pressure control valves. The pressure control valves hydraulically connect and disconnect the fluid pressure source to the fluid chambers of the hydraulic shock absorbers for controlling the pressure supply. The pressure control valves are controlled by an electric or electronic control system which switches the various valve positions to selectively introduce or drain fluid pressure into or from the fluid chambers so as to generate a damping force which suppresses vehicle body and road wheel vibrations.
These conventional positively controlled suspension systems encounter various defects and have not been at all satisfactorily capable of suppressing vibrations or bouncing of the vehicle body in order to ensure riding comfort. Specifically, conventional systems produce a damping force by means of an orifice in the hydraulic circuit. However, due to flow resistance through the orifice, fluid pressure differences between the fluid chambers in the shock absorber cannot be easily balanced. When the balance is disturbed, the shock absorber tends to transmit vibration of the road wheel to the vehicle body which degrades riding comfort.
In order to provide an improvement, a positively or actively controlled automotive suspension system was proposed in European Patent First Publication 01 93 124, published on Sep. 3, 1986, and assigned to the common owner of the present invention. The proposed positively controlled automotive suspension system comprises a hollow cylinder defining a chamber, a piston thrustingly received within the chamber of the cylinder and defining therein first and second fluid chambers, both filled with a working fluid the piston being free to move axially with the chamber, a fluid pressure source, a hydraulic circuit connecting the first and second fluid chamber and the fluid pressure source, a pressure control valve disposed within the hydraulic circuit and adapted to selectively establish and block fluid communication between the first and second fluid chambers and the fluid pressure source, means responsive to relative displacement between a vehicle body and road wheel assembly out of a predetermined normal range, for controlling the pressure control valve so as to adjust the fluid pressure in the first and second fluid chambers in order to adjust the relative distance between the vehicle body and the road wheel assembly back to within the predetermined normal range, and means responsive to bounding and rebounding motion of the road wheel relative to the vehicle body, for controlling the pressure control valve so as to adjust the fluid pressure in the first and second fluid chambers to assist smooth displacement of the piston within the cylinder thereby absorbing bounding and rebounding energy which would otherwise be transmitted to the vehicle body.
Another type of active suspension system has been disclosed in `Autocar` published by Haymarket Publishing Ltd., on Sep. 10, 1987. The disclosed system includes a single cylinder actuator which has a cylinder tube connected to the vehicle body and a piston with a piston rod connected to the suspension member. The cylinder actuator is connected to a hydraulic pressure source via an electromagnetic valve. The hydraulic cylinder is also connected to an accumulator via an orifice. With this construction, the pressurized fluid to absorb road shock and suppress attitude change of the vehicle body is supplied from the pressure source via the pressure control valve. This lower response of pressure control in the hydraulic actuator.
On the other hand, an automotive suspension system which has adjustable roll-stabilization ability was disclosed in the Japanese Patent First Publication (Tokkai) Showa 60-25201. The disclosed suspension system allows mechanical adjustment of the roll-stabilization ability by providing mechanical coupling in a roll-stabilizer. However, this adjustable roll stabilizer cannot preform precise and wide range roll-stability adjustment.